In order to optimize the illumination of a substance to be analyzed by spectroscope, there has been proposed an illuminator comprising a source of white light, such as an incandescent lamp (e.g. a halogen lamp) having a "dichroic" reflector, from which the beam of emitted/reflected light is transmitted, through an antiheat filter and an optical system of lenses, to the input of a sheaf of glass optical fibers, the output for which is situated in the proximity of the substance to be analyzed. This illuminator is provided with a forced cooling fan device for the lamp and for the parts adjacent thereto or heated thereby through irradiation.
However, this well-known illuminator displays the following principal drawbacks:
The dichroic reflector is provided for the reflection in the direction of the sheaf of optical fibers of just radiation in the visible spectrum, and in the opposite direction, through transparency, of a certain quantity of infrared rays, responsible for the heat, thus keeping these away from the area of application of the remaining light produced by the lamp. Accordingly, a reduced quantity of heat enters the sheaf of optical fibers. However, this dichroic reflector does not limit just the infrared rays, but generates absorption lines or strips or bands, albeit faint, in the visible region of the spectrum. Furthermore, as mentioned previously, the reflector-lamp group requires a forced cooling fan device, which is, inter alia, a source of continuous and irritating noise.
The antiheat filter, provided for the purpose of keeping back through absorption the infrared rays--responsible for the heat--in in the beam of light passing through the filter, offers the advantage of a partial absorption in the red and orange region starting from the commencement of the visible spectrum. These colors are in fact emitted in superabundance by incandescent lamps, and thus the filter partially "rebalances" the situation relating to the color temperature. Nonetheless, this absorption is rather greater than necessary. As a result, the residual light (i.e. the light intended for illuminating the substance) displays a dominant light green color.
The optical system of lenses entails a comparatively substantial technological and financial outlay. In the best possible case, the lenses consist of a glass, which apart from an unavoidable general absorption over the entire visible region, does not provide the residual light with a coloration that is particularly appreciable to the naked eye.
At its output, the sheaf of glass optical fibers supplies a type of light known as "cold" light (because of the actual absence of infrared rays from the emerging sheaf, rays which, as mentioned previously, are responsible for heat). In truth, this light ought to be called "unbalanced", inasmuch as it is notably lacking in blue and violet radiation, on account of a high degree of absorption in this region of the spectrum on the part of these optical fibers. Accordingly, the optical fibers in the main impart a further dominant light green color to the light transmitted by the optical fibers. In addition, from the quantity of luminous energy which these optical fibers receive from the optical system upstream, the optical fibers emit a beam of light, intended to illuminate the substance under examination, which is divergent, with a waste of energy and a reduction of the total luminous energy intended for the analysis.
It should also be noted that the human eye's sensitivity in the blue/violet region is extremely low, far more so than in comparison with the other colors of the spectrum. From this we infer the importance of the greatest possible "recovery" of energy in this region of the spectrum.
In addition to the above-mentioned drawbacks, it ought then to be borne in mind that owing to the requirements of design and construction, deriving from the principle upon which the instrument's operation is based, an exceptionally small quantity of luminous energy enters the spectroscope through a thin frontal slit. Frequently, in the use of the instrument, the shape and/or dimensions of the substance under examination are at variance with the tiny length of this slit, as shall become clearer from the following considerations relating, by way of example, to the possible types of gems et cetera to be submitted to spectroscopic analysis (for ease of exposition, the gem etc. shall hereinafter simply be referred to as "stone"):
Large stone having surfaces that are flat or rounded, but at any rate smooth; or else a large stone having relatively large facetted surfaces:
In these cases, there are no special problems inherent in observing the stone's absorption and/or emission spectrum.
Large stone having relatively small facetted surfaces:
Faces that are too small send to the spectroscope's slit bands of color having many and various directions and forms, and which, in particular, are not parallel among themselves. The observer sees a spectrum having more or less distinct patches or zones; the vision is disturbed, and any absorption or emission lines, bands or strips cannot be recognized as such; or else the limits between different unevenly illuminated zones may be mistaken for absorptions. The search for an optimal observation position becomes difficult, and may lead to unsatisfactory, contradictory or even misleading results.
Small stone:
The total light entering the spectroscope's slit consists of two separate fractions, i.e.:
a portion which has passed through the stone, and consequently having residual coloration properties governed by the nature of the stone passed through. This particular fraction is the fraction that is of use to spectroscopic analysis.
a portion reflected or transmitted by the base upon which the stone is resting, even in the event that this base is black. This portion is wholly devoid of information regarding the stone under examination.
In consequence, the observer sees a spectrum for the stone that is "diluted" in a quantity of more or less dazzling light. In this case, faint lines or bands disappear, which results in a lack of diagnostic features helpful in identifying the stone in question.
Large or small stone, and highly absorbent over the entire visible range:
stones that are highly absorbent over the entire region of the spectrum offer no opportunities for spectroscopic analysis other than at their edges (generally the thinnest edges) and/or with highly powerful illumination. In these cases we may refer to the foregoing drawback: a certain quantity of light that has not passed through the stone under examination enters the spectroscope's slit, with the same consequences as referred to above.